The present invention generally relates to a method and system for detecting atrial events of a heart. The present invention is more particularly directed to such a system and method which detects A waves of a heart in the presence of ventricular activity wherein A waves are normally detected with a first threshold but, during times in which ventricular activity is expected to occur, the threshold is raised to a second threshold, higher than the first threshold to avoid sensing ventricular activity as atrial activity.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life-threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience rapid and irregular beating of the heart and may even experience dizziness as a result of reduced cardiac output.
Atrial fibrillation occurs suddenly, and many times can only be corrected by discharging electrical energy into the atria of the heart of the patient. This treatment is preferably synchronized to a detected R wave of the heart in order to avoid shocking the atria during the T wave or vulnerable period of the heart. The amount of energy which may be required to successfully cardiovert the atria can be as low as one joule and as high as six joules. In most cases, energy of about two to four joules is required to cardiovert atrial fibrillation back to normal sinus rhythm (NSR).
Implantable atrial defibrillators are known which detect the presence of atrial fibrillation and provide a single cardioverting pulse of electrical energy to the atria when atrial fibrillation is detected. Usually, the therapy is applied in synchrony with a detected R wave to avoid therapy application during the ventricular vulnerable period of the heart thereby preventing the induction of a lethal ventricular arrhythmia.
Atrial fibrillation detection may be initiated at spaced apart times with such devices to conserve battery power as disclosed, for example, in U.S. Pat. No. 5,464,432. Alternatively, such devices may provide continuous monitoring of heart activity to activate more specific atrial fibrillation detections when the monitored activity indicates a probability of atrial fibrillation.
One such atrial defibrillator is disclosed in U.S. Pat. No. 5,282,837. As disclosed in that patent, ventricular activity is continuously monitored. When the ventricular rate and/or ventricular rate variability reach a certain level, atrial fibrillation is suspected and a more robust and higher battery energy consumption algorithm for atrial fibrillation detection is initiated and implemented with a microprocessor.
Continuous monitoring of ventricular activity to predict when atrial fibrillation may be present may be effective for many patients. However, for patients with heart block, ventricular sensing may not be as effective as would be desirable. The reason for this is that when a patient has heart block, ventricular activity is generally not associated with or related to atrial activity. Hence, ventricular activity may not provide the best indication of possible atrial fibrillation.
In view of the foregoing, it would be desirable to be able to continuously monitor atrial activity for identifying possible atrial fibrillation. Such monitoring, would be effective for all patients, including heart block patients. However, atrial sensing for purely atrial events can be complicated by the presence of other heart activity, such as ventricular activity. Far field sensing of ventricular activations (R waves) could be mistaken for atrial activity resulting in false indications of possible atrial fibrillation. The present invention provides a system and method of detecting atrial events which avoids detection of R waves as atrial events to result in accurate predictions of possible atrial fibrillation.